Feature/4717 accessibility improvements aria

[robinmolen]

Closes #4717

Changes

The site logo now provides a better textual alternative, the error message element now has the appropriate role and the PDF's are now screenreader friendly :)

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • [X] Checked copying a form
  • [X] Checked import/export of a form
  • [X] Config checks in the configuration overview admin page
  • [X] Problem detection in the admin email digest is handled

• Release management

  • [X] I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • [X] Ran ./bin/makemessages_js.sh
  • [X] Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • [X] Commit messages refer to the relevant Github issue
  • [X] Commit messages explain the "why" of change, not the how

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.55%. Comparing base (4c53505) to head (b5daa22). Report is 7 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4747 +/- ## ======================================= Coverage 96.55% 96.55% ======================================= Files 748 748 Lines 25413 25417 +4 Branches 3358 3361 +3 ======================================= + Hits 24538 24542 +4 Misses 610 610 Partials 265 265 ```

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